TurboTommy

Decided to install a cross-over back pressure test kit.
Tapped off the factory port in the cross-over pipe.
Ran pressure gauge into the cabin.

I did not do a full boost test; just went to about 8 or 9 psi boost because of a temporary tuning issue. Witnessed readings that shocked me! Accelerating moderately in second or third gear, and still slight manifold vacuum, I would already be seeing 5 psi exhaust back pressure. At very little boost (maybe 1 or 2 psi), I would get readings of over 10 psi back pressure at higher RPMs. At the 8 psi boost, back pressure was about 14 or 15 psi.

I'm supposed to have a fairly free flowing hotside. Is this normal? At these lower boost levels, we would assume that the wastegate is not coming into play yet, so, maybe this is the reason?
I will definitely report back when I've done the full boost test. But something doesn't seem right. How can there be that much resistance through the turbine?

Any gurus out there that understand the relationship between turbines, turbine pressure ratios, and back pressure?
I thought I had a handle on this stuff, but now I'm totally disallusioned.

Vitesse stage 5 turbo, by the way.

951_RS

I'm no expert by any means, but I would think that until the turbo starts spinning more it should have a bit of pressure. I would think this is because the exhaust gases are being forced into the exhaust manifold and there's no way for that pressure to be immediately removed as it's forced in and nothing is sucking it out so there must be pressure, which is what causes the exhaust to be pushed out to begin with. Once the wastegate opens a lot of the pressure will be released, and when the turbo is spinning faster perhaps it will not restrict the exhaust gases as much. I don't know I'm just talking right now =]

gt37vgt

although that turbo is made from regular garret and KKK bits course it is a "stage 5"
so no insight what so ever to what it is ?
i was under the understanding that interns of pressure ratios exhaust/ inlet
that 1.9:1 was normal for a road car 1.5:1 is good and 1;1 or better is F1 witchcraft voodoo.
not many people measuring exhaust manifold pressure as its not easy change turbo..
if you had a high mount garret you could bolt 5 different turbos on there in a few days and have data on them
but as its too hard to realty make changes most of us don't want to know ...

what would be worth while is if a few people get involded make 2 or 3 kits to fit that port and pass them around and share the results ..

thingo

If you lower the pressure after the turbo you can get a lower turbine pressure, it is on the other side of the 'nozzle' but it is still part of the system.

gt37vgt

you mean Big a exhaust does help reduce pre turbine pressure .

thingo

Seems to be the case with our cars more than others, probably a by product of the length of the crossover.

951_RS

That works for any turbo car. That is the reason bigger exhausts have such a large effect on turbo cars.
Reducing post turbo back pressure increases spool time and reduces pre turbine pressure which, simply put, produces more horsepower at the same psi level.

Rogue_Ant

Yes, but not as important as the turbine side.

Frankly, without turbo specs this thread is worthless.

That said, OEM stuff can be up to 2.5:1.
Ideally you want as close to 1:1 as possible, but not going to happen on a street car without some serious tricks.
1.5:1 is quite reasonable, and with the minimal overlap on the stock cam, I wouldn't be too worried about backpressure.


-Rogue

A.Wayne

The ideal ratio is 1.5:1 , even with a open short pipe exhaust you will still see 1.5:1 or higher . 2 Low a back pressure more lag, you balance the 2 for power and response .... 1.1 would be useless IMO on a street car unless running variable vane, yeah maybe with variable vane..

gt37vgt

yeh I'm trying for 300+ft/lbs from 2700 to 5700 with a GT35vgt 3.0 was thinking of shooting for 1.4:1 pressure ratios .

thingo

You should be able to get it lower than that, without too much trouble, but you will have to measure it to find out.

ehall

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gt37vgt

ah dunno Rod its not infanitly variable AR the hotside does have a .82 ar cast on it still not sure if i will set it up with a waste gate or not ..

Chris White

Infant variable Rod…??? Hot side? Wastegate?? What kind of web site is this???


Oh, yeah, those are on my other ‘favorites’ list…

Chris White

Ok, back to the issue…

I have never measured it but now I think I should – I would guess that on initial acceleration (turbine not up to speed, wastegate not open) you should see some higher than normal ratios. The conventional thoughts on pressure ratios are during steady state operation.